Two-phase step motors have been very popular in the motion control industry. With reference to FIGS. 1A and 1B, only the stator poles and coils of such a motor of the prior art are shown. The rotor is not shown. In actual implementation, the rotor will appear as a multi-tooth gear and the stator poles will have corresponding teeth so as to permit many motor steps per revolution. As shown in FIG. 1B, stator 15 has eight stator poles 11. Each stator pole 11 has a plurality of teeth 13. The first, second, third, and fourth stator coils are respectively labeled a.sub.1 a.sub.1 ', a.sub.2 a.sub.2 ', b.sub.1 b.sub.1 ', and b.sub.2 b.sub.2 ', or in alternative, SC1, SC2, SC3, and SC4, respectively. The first and second stator coils SC1 and SC2 are paired (bifilar) throughout the winding. The third and fourth stator coils SC3 and SC4 are also paired (bifilar) throughout the winding. These four stator coils are wound around the eight stator poles 11. The term "stator coil" used in this application refers to an electric wire wound around one or more poles 11 of the stator in a specified manner. As shown in FIG. 1A, each stator coil is a wire wound around four stator poles in alternating direction: counterclockwise and clockwise. Each stator coil is wound around every other stator pole 11. "Bifilar" winding pattern means that the stator coils are wound around the stator poles in pairs.
FIG. 1C shows the circuit diagram and FIG. 2 shows the electrical connection diagram for the typical two-phase 8-stator pole stator in FIG. 1A. With reference to FIGS. 1C and 2, the end a.sub.1 of the first stator coil SC1 in FIG. 1C forms the terminal 30 in FIG. 2. The end a.sub.1 ' of the first stator coil SC1 and the end a.sub.2 of the second stator coil SC2 in FIG. 1C are connected to form a center-tap 32 in FIG. 2. The end a.sub.2 ' of the second stator coil SC2 in FIG. 1C forms the terminal 34 in FIG. 2. Similarly, the end b.sub.1 of the third stator coil SC3 in FIG. 1C forms the terminal 36 in FIG. 2. The end b.sub.1 ' of the third stator coil SC3 and the end b.sub.2 of the fourth stator coil SC4 in FIG. 1C are connected to form another center-tap 38 in FIG. 2. The end b.sub.2 ' of the fourth stator coil SC4 in FIG. 1C forms the terminal 40 in FIG. 2. Terminals 30 and 34 connect to phase A of a two-phase driver. Terminals 36 and 40 connect to phase B of the two-phase driver. Phases A and B of the two-phase stator have a 90.degree. phase shift from each other. A reversed current through phase A (the first and second stator coils SC1 and SC2) having a 180.degree. phase shift from phase A is designated as phase A. A reversed current through phase B (the third and fourth stator coils SC3 and SC4) having a 180.degree. phase shift from phase B is designated as phase B.
The number of mechanical phases are two times the number of electrical phases in a bipolar device. The step resolution (the number of steps per revolution) of a step motor is determined by multiplying the number of rotor teeth by the number of mechanical phases.
An object of the present invention is to provide a connection scheme for a two-phase stator of a step motor so that the motor can be run more smoothly and with higher resolution (higher number of steps per revolution) by a three-phase driver.